Passive low frequency RFID readers and tags use operating principles that are well-know to those of ordinary skill in the art, and that are described in extensive detail in several seminal inventions, including U.S. Pat. No. 1,744,036 to Brard, U.S. Pat. No. 3,299,424 to Vinding, U.S. Pat. No. 3,713,148 to Cardullo et al., and U.S. Pat. No. 5,053,774 to Schuermann et al., and in textbooks such as Finkenzeller, “RFID Handbook” (1st Edition, 1999). The disclosure of U.S. Pat. Nos. 1,744,036, 3,299,424, 3,713,148 and 5,053,774 and the “RFID Handbook” is incorporated herein by reference in its entirety
In RFID systems of this type, the reader (also sometimes referred to as a scanner or interrogator) device generates a tag activation signal, and receives identification data signals from the electronic identification (EID) tag. Such a reader device can use separate transmit and receive antenna elements to perform these functions. Readers in which a single antenna performs both transmit and receive functions are typically economical and efficient, and are commonly used in low-frequency RFID reader designs.
An example of the circuitry of a conventional RFID system capable of reading EID tags is shown in FIG. 1. The reader includes circuitry, which generates an activation signal (usually a single frequency unmodulated signal) using a signal source 101 and an amplifier 102 to drive a resonant antenna circuit 103. This activation signal manifests as a time-varying electromagnetic field, which couples with the EID tag 105 by means of the electromagnetic field's magnetic field component 104. The EID tag 105 converts this magnetic field into an electrical voltage and current, and uses this electrical power to activate its internal electronic circuitry. Using any of several possible modulation schemes, the EID tag conveys binary encoded information stored within it back to the reader via magnetic field 104, where the detector and utilization circuit 106 converts this binary code into alphanumeric format tag data 107 in accordance with some prescribed application.
The resonant antenna circuit 103 in FIG. 1 typically includes at least one inductive and one capacitive circuit element. Examples of resonant antenna circuits are shown in FIGS. 2 and 3. In each circuit, the capacitive element 201 is connected to the inductive element 202. Functionally, the inductive element 202 radiates a magnetic field when driven by a time varying signal. Conversely, when exposed to a time varying magnetic field, current flow is induced in the inductive element 202, resulting in a time varying signal that appears at the resonant antenna circuit's connection points. These phenomena are well known to be consistent with a principle of physics known as Faraday's Law.
FIG. 2 illustrates a series-wired arrangement of the inductor 202 and capacitor 201, and FIG. 3 illustrates a parallel-wired arrangement of these same components. Either arrangement may be used in an RFID reader depending on other design attributes, such as the configuration of Amplifier 102 and/or the Detector and Utilization Circuit 106. In either case, the selection of inductor and capacitor values are determined by the equation FR=½π√LC, which is well known to those of ordinary skill in the art. For example, if the capacitance C of the capacitor 201 is 5 nF (i.e., 5×10−9 Farads) and the inductance L of the inductor 202 is 281 uH (i.e., 281×10−6 Henries), then the resonant frequency is expected to be 134.27 kilohertz (KHz). A variety of values for L and C can be combined to produce a particular resonant frequency FR, and the selection of specific values generally depends on other design considerations.
The packaging of an RFID reader can be important to the usefulness of the reader. Many RFI) readers are packaged for static mounting on a wall or in other fixed locations. Other types of readers are transportable in that they are stationary during use and easily moved for use where needed. Still other types of RFID readers are packaged as portable devices to be used to manually read EID tags. FIG. 4 illustrates in cutaway view a typical portable RFID reader 400 that could be used in a variety of possible EID tag reading applications. This RFID reader includes a non-metallic enclosure housing 401 having a handle grip 406, and which encloses the reader components, including a ferrite antenna 800, the reader's electronic circuitry 402, a battery power source 405, an activation switch 403, indicator lights 404, and antenna connecting wires 407. Each end of the RFID reader's packaging is sealed with an end cap 408. The RFID reader 400 includes a capacitor similar to the capacitor 201 shown in FIG. 2 as part of its electronic circuitry 402.
FIG. 4 also illustrates a typical EID tag 409 and an RFID reader 400. The EID tag 409 is shown oriented with respect to a ferrite antenna 800 of the RFID reader in a way that is optimal for activation and reading. The ellipses depicted by the dashed lines 410 are indicative of a magnetic field coupling the ferrite antenna 800 with the EID tag 409. The magnetic field conveys an activation signal to the EID tag 409 and a data signal to the ferrite antenna 800.
In many applications, providing antenna extensions to vary the length of a portable RFID reader can enhance the usefulness of the portable RFID reader. FIG. 5 illustrates a portable RFID reader that can be combined with various antenna extensions to vary the RFID reader's antenna length. The portable RFID reader 500 includes an RFID reader base 501a and one or more antenna extensions 501b. The RFID reader base 501a and an antenna extension 501b combine together to form a complete portable RFID reader. Although not shown in FIG. 5, the RFID reader base 501a includes all the components of the reader 400 shown in FIG. 4, except that the RFID reader base 501a does not contain an antenna. Instead, the end of the RFID reader base 501a is equipped with an electromechanical coupler 504a, which includes a mechanical joining mechanism 505a and an electrical connector 506a. The electrical connecter 506a has wiring 507 that electrically connects the connector 506a to the reader's electronic circuitry.
The antenna extension 501 contains a ferrite antenna 502 whose electrical wires 503 are routed through the enclosure to electrical connector 506b. Connector 506b is part of electromechanical coupler 504b, which also includes mechanical joining mechanism 505b. Electromechanical coupler 504a is designed to electrically and mechanically mate with electromechanical coupler 504b, such that electrical connector 506a mates with electrical connector 506b, and the mechanical joining devices 505a, 505b also mate at junction point 508. The antenna extension 501 can be any length that is of practicable use. The electromechanical couplers 504a, 504b are illustrated conceptually rather than in detail, inasmuch as many variants of the same coupling principle can and do exist, but all function to a greater or lesser extent as described above.
Although the reader antenna extension shown in FIG. 5 and described above is functional, the electromechanical coupler can be expensive and is prone to electrical and mechanical fatigue and failure. The electromechanical coupler can also require alignment of mating parts, a task that can be difficult to perform and which can impart damage to the associated components if executed incorrectly. The electromechanical coupler can also become contaminated with foreign debris, which can damage the coupler's integrity and impair its functionality. In addition, the reader assembly must be specially equipped with the electromechanical coupler, differentiating it from a standard reader that has an integral antenna.